For oil wells and gas wells (hereinafter, referred simply to as “oil wells”) in a deep or severe corrosive environment, high alloy pipes made from a high Cr-high Ni alloy have been used as oil well pipes. For the purpose of use in an environment that is severer than the conventional environment, a high-strength high alloy pipe having a strength especially of as high as 110 to 140 ksi grade (minimum yield strength: 757.3 to 963.8 MPa) and also having corrosion resistance has been demanded. In the case where the high-strength high alloy pipe is used as an oil well pipe in an environment in which a bending force or a tensile force is applied, both of strength and high ductility have been required because buckling, breakage, and the like may occur. For example, ISO 13680 “Petroleum and natural gas industries—Corrosion-resistant alloy seamless tubes for use as casing, tubing and coupling stock—Technical delivery conditions” specifies that elongations at the yield strengths of 110 ksi grade (757.3 MPa), 125 ksi grade (860.5 MPa), and 140 ksi grade (963.8 MPa) should be 11% or higher, 10% or higher, and 9% or higher, respectively. Thus, for the purpose of use in a severer environment, a high alloy pipe having a further high elongation has been demanded.
Furthermore, from the viewpoint of manufacture, the high alloy pipe is manufactured from a high alloy billet in hot working processes by extrusion pipe making processes including the Ugine-Sejournet process, the Mannesmann pipe making process, or the like. Excellent hot workability is also required in such processes.
Patent Documents 1 and 2 disclose an austenitic stainless steel in which, in order to prevent intergranular cracking from occurring when a high alloy steel cast piece manufactured by continuous casting is hot rolled, the hot workability is improved by controlling the S content and the O content to a range defined by an expression in relation to the Ca content and the Ce content. However, no material design in which the improvement is taken into consideration of ductility at the time when the high Cr-high Ni alloy is subjected to the final cold working process to strengthen the alloy has been studied.
On the other hand, Patent Documents 3 to 6 disclose a method for obtaining a high-strength high alloy oil well pipe by subjecting a high Cr-high Ni alloy to hot working and solution treatment and then to cold working at a wall thickness reduction ratio of 10 to 60%.
Furthermore, Patent Document 7 discloses an austenitic alloy excellent in corrosion resistance in a hydrogen sulfide environment, which is cold worked by controlling the shapes of inclusions with La, Al, Ca and O contained in a specific relation. The cold working in this invention is performed to give strength; from the viewpoint of corrosion resistance, the wall thickness reduction ratio is defined as 30% or less.
Also, Patent Document 8 discloses a high Cr-high Ni alloy in which the contents of Cu and Mo are adjusted to improve the SCC resistance in a hydrogen sulfide environment, and describes that it is preferable that the strength be controlled by further performing cold working at a working ratio of 30% or less after hot working.
Patent Document 1: JP59-182956A
Patent Document 2: JP60-149748A
Patent Document 3: JP58-6927A
Patent Document 4: JP58-9922A
Patent Document 5: JP58-11735A
Patent Document 6: U.S. Pat. No. 4,421,571A
Patent Document 7: JP63-274743A
Patent Document 8: JP11-302801A